In magnetic data recording devices such as disk drives and tape drives, a magnetic medium is typically formatted into sectors. Sectors are typically the smallest addressable units of dam. The magnetic properties of a particular magnetic medium typically vary considerably over the surface of the medium. After formatting, each sector is typically tested for defects by verifying that the magnitude of resulting data signals exceeds some threshold value. Any sector which does not qualify is marked as unusable.
In devices with unremovable media such as rigid disk drives, the quality measure for sector verification can include the combination of drive and media. That is, both the electronics and the media can vary and what matters is the performance of the combination. However, for removable media devices such as flexible disk drives and tape drives, the drive and the medium must each meet independent absolute minimum specifications to ensure interchange between drives. Industry specifications for interchange require each drive to have the ability to verify that a newly formatted medium conforms to an absolute media specification.
An example industry specification for interchangeable tapes is the QIC-80 Development Standard (Revision I, Sep. 2, 1992, available from Quarter-Inch Cartridge Drive Standards, Inc., 311 East Carrillo Street, Santa Barbara, Calif. 93101). This standard provides for a reference tape cartridge which can be used to calibrate drives. When a tape cartridge is exchanged between calibrated drives, a particular magnetic transition should result in a standard signal amplitude at a standard point in the drive. During verification, any sector having a read signal which drops below 47% of a standard amplitude is eliminated as a defective sector.
Integrated circuits are commercially available which contain much of the circuitry used for reading magnetically recorded data. It is desirable to also use such circuits for media verification in addition to data reading. However, in general, the data reading circuitry may not provide the precision and absolute standards needed to meet industry precision standards for media verification. The present invention provides precise verification with commercially available parts which do not inherently provide the necessary precision. The following discussion provides additional technical background for the present invention.
In a typical magnetic data recording device, binary data is recorded along a track in a magnetic medium by alternately magnetizing small areas from one magnetic polarity to the opposite polarity. The data is encoded in the timing of the polarization reversals, not in the polarity of magnetization. The process of reading typically employs a magnetic head which has a voltage output which is proportional to the rate of change of a magnetic field. For data, the rate of change of the magnetic field (and corresponding voltage) is highest at a boundary where the magnetic polarity reverses. Therefore, the data which was encoded in the timing of magnetic reversals during recording is encoded in the timing of signal peaks during reading. Rather than detect the timing of peaks, the voltage signal is typically differentiated so that peaks in the non-differentiated signal become zero crossings in the differentiated signal. Therefore, in the differentiated signal, the data is encoded in the timing of zero crossings.
With noise, there may be transient zero crossings in the differentiated signal which do not correspond to a magnetic polarity reversal. One solution to help distinguish valid signals from noise is to use a dual path detection system. One path uses the original non-differentiated signal and the other path uses the differentiated signal. In the non-differentiated signal path, the voltage peaks are compared to a predetermined voltage threshold using an analog comparator. The comparator output in the non-differentiated path is used to qualify zero crossings in the differentiated path as follows. During the time window that a voltage peak in the non-differentiated path is opposite in polarity to the previous peak and greater in magnitude than the threshold, any zero crossings in the differentiated path are assumed to be valid. If however the peak voltage in the non-differentiated path is of the same polarity as the previous peak or has a magnitude below the threshold, any zero crossings in the differentiated path during that time are rejected as noise.
Circuitry providing dual path detection with qualification as described above is contained within commercially available integrated circuits. For example, the SSI 32P541 Read Data Processor (Silicon Systems Inc., 14351 Myford Road, Tustin, Calif. 92680) contains circuitry for performing the functions described. In addition, there are compatible parts from multiple other vendors. In this compatible family of commercially available read data processor circuits, the comparator in the non-differentiated path has hysteresis. The circuits have an external hysteresis input for controlling the amount of hysteresis. The comparator hysteresis provides two thresholds, one for each polarity of peaks. If a peak exceeds one threshold, the hysteresis switches the threshold to the opposite polarity so that only an opposite polarity peak can toggle the comparator output. These commercially available circuits also provide a rectified signal output.
The design approach suggested by the vendors of the commercially available read data processors is to filter the rectified signal output to provide a peak detector and to use a fixed fraction of the peak detector voltage to control the hysteresis level of the comparator. Ideally, the comparator threshold level is then a fixed percentage of the absolute value of the peak signal voltage, varying linearly as the peak signal level varies.
For media verification, there are several problems which are not solved by the commercially available integrated circuits discussed above. First, with an unknown medium being verified, there is no standard amplitude signal available. Second, the actual threshold voltage resulting from a particular fixed hysteresis input voltage varies from part to part. This variation is acceptable for the intended function of qualification of zero crossings. However, more precision is needed if these parts are to be used for verification.